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"Which" implies that you have a fixed set of choices: "I have strawberry, banana, and blueberry muffins. Which kind would you like?" In this case, "which" is tied very closely to the muffin choices. Without option from which to choose, "which" cannot be used.

"What", on the other hand, is more versatile, and it seems to be used more often in spoken English. For example, you could also use "what would you like?" in the scenario above. In that case, "what" is not tied specifically to the kinds of muffins, but is more general (but the meaning is roughly the same). You can also use "what" without set options - it can be very open-ended. "What would you like?" on its own gives you the opportunity to name anything.

Here is a final example:1.) What are you doing?The person can answer anything, so you can start a conversation like this without any prior knowledge. "Which" doesn't work here without an established set of options.

2.) Which are you doing?You can only ask this if you already know the options, so you have to be familiar with the situation at hand. If you already know your friend could go to the movies or go out to lunch, you could ask him "which are you doing?"